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Cattle were fed chlorpyrifos daily for 30 days at levels of 3,10, 30, and 100 ppm. Muscle, liver, kidney, omental fat, renal fat, and subcutaneous fat were collected at the end of this period. In addition, omental fat was collected by biopsy at weekly intervals for 5 weeks following withdrawal of the highest level of 100 ppm chlorpyrifos. Residues of chlorpyrifos and its oxygen analogue were determined by thermionic or flame photometric gas chromatography. The 3,5,6-trichloro-2-pyridinol moiety as the trimethylsilyl derivative was determined by electron-capture gas chromatography. The procedures were used to quantitate chlorpyrifos and its oxygen analogue down to 0.01 ppm and 3,5,6-trichloro-2-pyridinol to 0.05 ppm. Residues of chlorpyrifos were mainly in the fat tissues and averaged 0.02 ppm (<0.01-0.05 ppm) with 3 ppm in the diet and 3.28 ppm (2.28-4.70 ppm) in fat of cattle fed 100 ppm. The 3,5,6trichloro-2-pyridinol was predominantly in the liver and kidney and averaged 0.20 ppm (0.16-0.23 ppm) in liver and 0.11 ppm (0.09-0.15 ppm) in kidney at 3 ppm; 2.41 ppm (2.16-2.61 ppm) in liver and 1.75 ppm (1.46-1.95 ppm) in kidney at the 100 ppm feeding level. No chlorpyrifos oxygen analogue was detected in any tissue at any feeding level.DURSBAN, trademark of The Dow Chemical Company

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A gas chromatographic method for determining residues of O,O-diethyl 0-3,5,6-trichloro-2-pyridyl phosphorothioate (Dursban insecticide) in water and silt is described. Samples were extracted with methylene chloride and cleanup was accomplished with a silicic acid column. Residues as low as 0.0001 p.p.m. in water and 0.005 p.p.m. in silt were determined by gas chromatography using nonpolar stationary phase columns and electron-capture

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